Rats in the rice

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If you're having problems with mice in the muesli, spare a thought for rice farmers in southeast Asia, who lose an average of 15 per cent of their crop to rats. Many resort to harmful chemicals - but now ecologists are developing better solutions.

Research by Australia's CSIRO Sustainable Ecosystems Rodent Group has found that rats travel further, and breed more quickly, than has been thought - explaining why individual farmers' control efforts are failing.

"If you're a farmer doing everything right with your one hectare plot, it's not enough," said Dr Grant Singleton, head of the Rodent Group, currently based at the International Rice Research Institute in the Philippines. "You need a system that protects your neighbour's field as well as your own."

The scientists have used radio collars to track the movements of the main problem species, the rice-field rat (Rattus argentiventer), in west Java, Indonesia. They were surprised at the distances travelled.

"It's not unusual for rats to move 200 metres in one night, and sometimes up to half a kilometre," said Dr Singleton. "They're very mobile."

The results mean that many of the methods used by individual farmers, such as plastic fences around their plots, do little but redivert rats to neighbouring fields - where they continue feeding and breeding.

Meanwhile, farmers are planting more rice crops each year, in order to feed growing human populations. This gives the rats a more continuous food supply and therefore longer breeding cycles - during which the young themselves are able to breed, rapidly escalating total rat numbers.

"The rats are eating enough rice to feed more than 20 million Indonesians for a whole year," said Dr Singleton. "And that's before the rice has even been harvested."

Desperate measures

The rats' growing appetites have lowered farmers' tolerance for sharing their crops. Their reported rule of thumb is that 'for every eight rows of rice we sow for our family, we sow two for the rats'.

Some farmers use chemical rodenticides, but according to Grant Singleton, these are often imported with instructions in Chinese and are therefore misused, killing cats, dogs, and chickens.

Researchers are now working with farmers to develop solutions based on both rat biology and economic need.

One idea is the "lure crop" - a rice field planted every 10 hectares in advance of the main crop, designed as bait for rats. The crop is then enclosed by a fence of plastic sheeting, and traps placed outside holes in the plastic to catch rats as they try and leave.

It is also important for all the farmers in a particular area to synchronise planting times and fallow periods, to interrupt the rats' food supply and breeding cycles.

Both solutions require community cooperation, said Dr Singleton, meaning that what works in one place won't necessarily work in another.